1. Field of the Invention
This invention relates to fuel gaging systems, and is particularly applicable to aircraft having a plurality of fuel tanks.
2. Description of the Prior Art
Known prior art gaging systems employed in currently operating aircraft typically employ a plurality of probes in each of the separate fuel tanks to develop signals indicating the volume of fuel in each tank. These probes comprise capacitors which are connected so that their admittance varies linearly as an analog function of quantity of fuel in the tank. One example of a typical bridge-type capacitance probe and gaging system may be found in U.S. Pat. No. 2,638,000 of Sontheimer. A compensating capacitor, totally submerged in the fuel, provides another analog indication that varies with the density of the fuel. These signals are sometimes directed to master indicators in the cockpit panel which include various circuitry to convert the analog signals to digital outputs which drive displays. Signals from this circuitry are also sometimes directed to repeater displays in the refueling panel. One such system, developed for an aircraft with three fuel tanks, is described in detail in U.S. Pat. No. 3,830,090 of Hersch et al. Related systems for measuring the capacitance of fuel tank probes and for testing the accuracy of the system by comparison to known reference signals are disclosed in Horowitz U.S. Pat. No. 3,801,902 and in Rubel et al U.S. Pat. Nos. 4,080,562 and 4,147,050. The disclosures of those patents are incorporated here by reference as though set forth in full herein. The Rubel et al system uses triangular wave test signals for improved accuracy and increased immunity to spurious coupling and line loading variations.
Other fuel gaging systems for aircraft are known in the prior art; for example, U.S. Pat. No. 2,697,348 of Bevins discloses such a system which totalizes the fuel consumption of a plurality of engines and provides a display indication of the remaining fuel in the tanks. Doughety et al. U.S. Pat. No. 4,258,422 discloses another fuel gaging and totalizing system.
The problem of adequately displaying the considerable amount of information relating to the condition and operation of an aircraft in a manner which can be readily monitored and assimilated by the pilot is recognized in U.S. Pat. No. 3,665,439 of Brummer et al. and U.S. Pat. No. 3,906,437 of Brandwein et al. Brummer et al disclose a supervision system for aircraft which utilizes multiplexing of signal information over a signal transmission link between measuring devices and cockpit display units. The system of Brandwein et al. involves rather intricate analog signal measurement and conditioning with further incrementing and application to an analog display and the additional provision of a selector switch for applying particular signals to an analog-to-digital converter and an associated digital display. An objective of the system is to develop an alarm in the event of deviation of measured signal conditions from some norm or threshold reference level. Owens, Jr. et al. in U.S. Pat. No. 3,626,398 similarly disclose a multiple display system for aircraft, simultaneously displaying data corresponding to present condition values and data corresponding to deviation of those conditions from a preset reference. A multiplexing signal processing system with cathode ray tube display is disclosed in U.S. Pat. No. 3,248,650 of Bialkowski et al.
With all of the effort that has been directed to providing simplified and accurate display of aircraft operating in flight conditions to the pilots, problems still remain. This is particularly true in the case of fuel gaging systems in commercial aircraft in which fuel quantity information is derived from a plurality of fuel tanks and displayed to the pilot in terms of total fuel and individual fuel tank quantities. It is clear that inherent inaccuracies in present gaging systems place limits on effective fuel capacities with resultant burdens on operating economy, aircraft range and the like.
The problem has become more severe in recent years with increased variation of fuel properties. Present gaging systems depend for accuracy on precise control of the density of the fuel stored. Because of the variation in quality control in the fuels provided in some airports and because of fuels being refined from new sources of crude oil, there is a much greater variation encountered in the fuel density. The typical variation leads to a three or four percent error in fuel quantity measurement. The DC 10 airliner, for example, carries 240,000 pounds of fuel; thus maintaining a reasonable reserve requires carrying at least 10,000 pounds as unused dead weight.
The errors in capacitance measurement of fuel quantity resulting from the variation in fuel density which is encountered may be reduced by the use of densitometer devices. A number of liquid densitometers are known in the prior art, as exemplified by U.S. Pat. Nos. 2,934,476 of Bernstein, 4,007,627 of Stansfeld, and 4,256,403 of Powell. Such densitometers have not as yet been incorporated in known fuel gaging systems for aircraft. However, the need for densitometers in fuel quantity measurement systems has become more critical with the above-noted increased variation in fuel density in order to improve the accuracy of the measurements.